Uncertainties of Extracting Amplitude and Frequency of Orbitally Driven Sea-Level Fluctuations from Shallow-Water Carbonate Cycles
Eberli, Gregor P.
In the absence of an accurate age model high-resolution chronostratigraphy of carbonate successions often relies on spectral analyses of cycle stacking patterns and the comparison to the frequencies of orbital forcing mechanisms. There is, however, overwhelming evidence in the modern and the Pleistocene of Great Bahama Bank that carbonate depositional cycles are unreliable recorders of both the frequency and the amplitude of orbitally driven sea-level fluctuations. This unreliability causes large uncertainties for forward modeling of reservoir units in shallow-water carbonates.
Uncertainties in the assessment of the amplitude are caused by unfilled accommodation space and the inability to measure the amount of sea-level fall in platform-top sediments. Unfilled accommodation space is not recorded in the strata. Likewise, the amount to which sea level drops below an exposure surface is not recorded in the stacked shallow-water cycles. As a consequence, carbonate cycles record only a fraction of the amplitude of a sea-level cycle: a portion of the rise and nothing of the fall.
Uncertainties in the frequencies are caused by the variable amplitude of sea-level change, producing "missed beats" on the platform, and meter-scale oscillations of sea level within highstands that potentially produce cycles of very short durations. "Missed beats", where a sea level fluctuation is not recorded because the subsequent sea-level rise does not create accommodation space and a new cycle, occur preferentially during times of long-term sea-level fall but are also produced by the depositional topography that is created by the irregularly filled accommodation space. Furthermore, sea level amplitudes based on the isotope record document random amplitude variability during the last 57 glacio-eustatic changes, complicating a model-based solution for capturing missed beats. In addition, suborbital oscillation of the sea level during highstands with amplitudes of up to 15 m produce cycles of short durations. Suborbital, intermittent sea-level oscillations during the last interglacial (MIS 5e)are documented in the Bahamas in stacked reefs, in repetitions of beach-eolian successions, and by subaerial exposure surfaces on subtidal facies.
The combined effects of missed beats and oscillations within highstands are are difficult to extract from the rock record. Consequently, establishing a chronostratigraphic framework in carbonates based on astrochronology is full of uncertainty.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013